Alerting: Add setting to distribute rule group evaluations over time (#80766)

* Simple, per-base-interval jitter

* Add log just for test purposes

* Add strategy approach, allow choosing between group or rule

* Add flag to jitter rules

* Add second toggle for jittering within a group

* Wire up toggles to strategy

* Slightly improve comment ordering

* Add tests for offset generation

* Rename JitterStrategyFrom

* Improve debug log message

* Use grafana SDK labels rather than prometheus labels

pkg/services/ngalert/models/testing.go

@@ -186,6 +186,12 @@ func WithInterval(interval time.Duration) AlertRuleMutator {
 	}
 }
 
+func WithIntervalBetween(min, max int64) AlertRuleMutator {
+	return func(rule *AlertRule) {
+		rule.IntervalSeconds = rand.Int63n(max-min) + min
+	}
+}
+
 func WithTitle(title string) AlertRuleMutator {
 	return func(rule *AlertRule) {
 		rule.Title = title